The piezoelectric material is a material having both the piezoelectric effect that electric polarization changes upon application of external stress and the inverse piezoelectric effect that the application of an electric field produces a mechanical distortion. The piezoelectric material is used in sensors for measuring pressure and distortion, resonators, and actuators.
Most piezoelectric materials currently used in practice are ferroelectric materials having the perovskite structure including tetragonal or rhombohedral PZT (PbZrO3- PbTiO3 solid solution) materials and tetragonal PT (PbTiO3) materials. Distinct performance requirements are met by adding various auxiliary components to these materials.
However, many piezoelectric materials of the PZT and PT systems have a Curie point in the range of about 300 to 350xc2x0 C. as long as they have practically acceptable compositions. Since the currently used soldering step is generally at a temperature of about 230 to 250xc2x0 C., piezoelectric materials having a Curie point of about 300 to 350xc2x0 C. tend to deteriorate their properties during the soldering step. If lead-free solders are developed to the practical level, the temperature of the soldering step would become higher. It is thus very important for piezoelectric materials to have a higher Curie point.
Prior art lead base piezoelectric materials are undesirable from the ecological and pollution standpoints because they contain a substantial content (about 60 to 70% by mass) of lead oxide (PbO) which is highly volatile even at low temperatures. More particularly, when such lead base piezoelectric materials are prepared as ceramics and single crystals, heat treatments such as firing and melting are indispensable, upon which an amount of lead oxide will volatilize and diffuse into the air, the amount being substantial when considered from the industrial level. Lead oxide can be recovered as long as it is released in the manufacturing stages. However, few effective means are currently available for recovering the lead oxide in piezoelectric materials after delivery to the market as industrial products. If a substantial amount of lead oxide is released from such products to the environment, it surely becomes a cause of pollution.
One of well-known lead-free piezoelectric materials is BaTiO3 having the perovskite structure belonging to the tetragonal system. This material, however, is impractical since its Curie temperature is as low as 120xc2x0 C. JP-A 9-100156 describes (1-x)(Bixc2xdNaxc2xd)TiO3-xNaNbO3 solid solutions of the perovskite structure, none of which have a Curie temperature in excess of 370xc2x0 C.
Bismuth layer compounds are known as the piezoelectric material which can have a Curie point in excess of 500xc2x0 C. Regrettably, because of the lack of lead, bismuth layer compounds have a low Qmax which is crucial in the application to resonators. Here Qmax is tanxcex8max wherein xcex8max is a maximum phase angle. More specifically, provided that X is a reactance and R is a resistance, Qmax is a maximum of Q (=|X|/R) between the resonance frequency and the antiresonance frequency. The greater the Qmax, the more stable becomes oscillation. Also, oscillation at a lower voltage becomes possible.
The Preprint of the 16th Applied Ferroelectric Meeting (May 26-29, 1999), pp. 97-98, includes a report about a lead-free bismuth layer compound having improved Qmax. The lead-free bismuth layer compound described in this report is (Sr1-xMex)Bi4Ti4O15 wherein Me is Ba, Ca, La, Sm or Gd. Ba and Ca are added in a range of xxe2x89xa60.1; Sm and Gd are added in a range of xxe2x89xa60.4; and La is added in a range of xxe2x89xa60.5. In the report, Qmax was measured in the thickness extensional fundamental vibration mode. FIG. 2 in the report shows that Qmax is improved by the addition of La whereas Qmax is reduced when Ba or Ca is added.
An object of the invention is to provide a novel and improved piezoelectric ceramic material which is free from lead and has a high Curie point and improved piezoelectric characteristics.
In a first aspect, the invention provides a piezoelectric ceramic material comprising a bismuth layer compound containing MII, Bi, Ti and O wherein MII is at least one element selected from the group consisting of Sr, Ba and Ca, and containing MIIBi4Ti4O15 type crystals, wherein MII is represented by the formula: SrxBayCaz wherein x, y and z representing the atomic proportions of Sr, Ba and Ca, respectively, satisfy x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.9, and 0xe2x89xa6zxe2x89xa61. This piezoelectric ceramic material utilizes thickness shear vibration. Preferably, y is in the range: x/6+0.2xe2x89xa6yxe2x89xa60.8.
In a second aspect, the invention provides a piezoelectric ceramic material comprising a bismuth layer compound containing MII, Bi, Ti and O wherein MII is at least one element selected from the group consisting of Sr, Ba and Ca, and containing MII,Bi4Ti4O15 type crystals having a c-axis length of at least 41.00 xc3x85. This piezoelectric ceramic material utilizes thickness shear vibration. Preferably, the MIIBi4Ti4O15 type crystals have a c-axis length of at least 41.30 xc3x85 and also preferably, up to 41.80 xc3x85.
In a third aspect, the invention provides a piezoelectric ceramic material comprising a bismuth layer compound containing MII, Bi, Ti and O wherein MII is at least one element selected from the group consisting of Sr, Ba and Ca, and containing MIIBi4Ti4O15 type crystals, wherein MII is represented by the formula: SrxBayCaz wherein x, y and z representing the atomic proportions of Sr, Ba and Ca, respectively, satisfy x+y+z=1, 0xe2x89xa6x less than 0.9, 0xe2x89xa6yxe2x89xa60.9, and 0xe2x89xa6z less than 1. This piezoelectric ceramic material utilizes thickness extensional vibration. Preferably, y is in the range:
yxe2x89xa6xe2x88x920.8 x+0.9. 
Further preferably, z is in the range:
xe2x88x920.2 x+0.3xe2x89xa6z. 
In the above embodiments, the piezoelectric ceramic material preferably further includes a lanthanoid oxide wherein Ln represents the lanthanoid, and the atomic ratio Ln/(Ln+MII) is in the range: 0xe2x89xa6Ln/(Ln+MII)xe2x89xa60.5. The piezoelectric ceramic material may further include manganese oxide.
In a fourth aspect, the invention provides a piezoelectric ceramic material comprising a bismuth layer compound containing Ca, Bi, Ti, Ln and O wherein Ln is a lanthanoid, and containing CaBi4Ti4O15 type crystals, wherein the atomic ratio Ln/(Ln+Ca) is in the range: 0xe2x89xa6 Ln/(Ln+Ca) less than 0.5. The piezoelectric ceramic material may further include manganese oxide.